1. Field of the Invention
The present invention relates to temperature measuring devices, and more particularly to temperature measuring devices for remote temperature measurement by means of infra-red detectors used in cooperation with infra-red optical systems for focusing infra-red images, and visible light optical systems for alignment of the infra-red optical system with the desired object of measurement.
2. Prior Art
In the past, temperature measurement of moving, flowing, or remote objects has presented a difficult instrumentation problem which has not previously been satisfactorily resolved. Moving and inaccessible objects post a particularly vexing problem since attachment of measuring apparatus is not generally possible. Immersion in a fluid solves the problem for flowing substances at least at the point at which the sensor is immersed, but does not provide for measurement in differing locations.
For those situations in which it is possible to attach thermocouples, thermopiles, thermally sensitive resistors, semiconductor junctions, and similar devices, the problem is generally thought to have been satisfactorily solved. Even in many of these applications, however, the inconvenience of measuring temperature may be such that the effort is abandoned because it is not cost effective.
Attempts to make temperature measurement independent of contact with the object being measured led to the development of radiometric sensors which are sensitive to the infra-red radiation emanating from hot bodies. Apparatus dependent upon this principle has previously been devised for remote temperature measurements using optical systems to focus the infra-red energy on the detector for maximum sensitivity. Both hand-held portable apparatus and laboratory type instruments presently exist, as do air-borne and space-borne systems for terrain surveillance.
A common problem in hand-held instruments, and portable instruments which operate with varying spatial relationships between object to instrument, is the difficulty of accurate alignment of the detector with the exact area of interest. For aiming the device, simple mechanical sighting systems have been employed which suffice for many applications but suffer from the drawback that, in addition to being difficult to align and maintain in alignment, the observer does not know exactly how much of the object is being viewed by the detector or how much of it is contributing to the temperature indication. Generally, the observer must guess at this factor. Since it is more often than not desired to find temperature anomalies such as small "hot spots" or "cold spots", an instrument which views too large an area will not suffice. Bringing the instrument closer to the object either physically or optically will provide more sensitivity to small anomalies, but the observer still must guess how much of the scene is being viewed. Additionally, close-up use of the instrument generally brings a concomitant problem of parallax error so that the observer is led to conclude that an area other than the actual area of the anomaly is responsible for a hot or cold spot temperature.
Finally, sighting and reading of the instruments have in the past entailed two separate operations. While the observer views the object of interest through the instruments' sights, he must first align the instruments, then he must shift the focus of his eye from far to near and read the indication on the instruments' readout while holding the alignment previously established. For many users, the requisite steadiness is not attainable. As the field of view of the instrument is narrowed to increase the instruments' ability to resolve smaller areas, the problem becomes even more acute. Thus, a practical limitation on the useable resolution of hand-held instruments is quickly reached.